The present invention relates to a multi-stage turbocharger system. Particularly, but not exclusively, the present invention relates to a two stage turbocharger system.
Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.
In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between facing radial walls arranged around the turbine chamber; an inlet arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.
Another known approach to improving turbocharging efficiency for an engine with a wide speed/load range is to provide a sequential two stage turbocharging system, comprising one relatively small high pressure turbocharger and another relatively large low pressure turbocharger. The turbochargers are arranged in series so that exhaust from the engine flows first through the smaller turbine of the high pressure turbocharger and then through the larger turbine of the low pressure turbocharger. A valve controlled bypass passage is provided for allowing exhaust gas to bypass the high pressure turbine at high engine speeds and/or loads. Similarly, the compressors of the two turbochargers are also arranged in series, with air flowing first through the relatively large compressor of the low pressure turbocharger and then through the relatively small compressor of the high pressure turbocharger. Again, a valve controlled bypass is provided to allow the inlet air to bypass the compressor of the high pressure turbocharger at high engine speeds and/or loads.
It is an object of embodiments of the present invention to provide an alternative or improved multi-stage turbocharger system.